1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus equipped with the same. More particularly, the present invention is suitably applied to an image forming apparatus, such as a laser beam printer or a digital copying machine using an electrophotography process and a multifunction printer, in which image information is recorded by optically scanning a surface to be scanned by means of a scanning optical system having fθ characteristics with a light beam emitted from a light source and deflected by a polygon mirror serving as a light deflector.
2. Related Background Art
In scanning optical apparatuses such as laser beam printers, a light beam is modulated in accordance with an image signal and emitted from a light source, then is periodically deflected by a light deflector composed of, for example, a rotary polygon mirror (or a polygon mirror), and is converged to form a spot on a surface of a photosensitive recording medium (e.g. a photosensitive drum) by an fθ lens system having fθ characteristics to scan the surface of the recording medium, thereby image recording is conventionally performed (see Japanese Patent Application Laid-Open No. 2003-121773).
FIG. 10 is a cross sectional view schematically showing the principal portion of a conventional optical scanning apparatus. As shown in FIG. 10, a divergent light beam emitted from a light source means 71 is converted into a substantially parallel light beam or a convergent light beam by a collimator lens 73. Then, the light beam (or the light quantity) is shaped (or adjusted) by an aperture stop 72 and made incident on a cylindrical lens 74 having a refractive power only in the sub-scanning direction. The light beam incident on the cylindrical lens 74 is emitted from it with its state being unchanged with respect to the main scanning cross section but converged with respect to the sub-scanning cross section, and focused as a substantially linear image in the vicinity of a deflecting surface 75a of a light deflector 75 composed of a rotary polygon mirror (or a polygon mirror).
The light beam reflected and deflected by the deflecting surface 75a of the light deflector 75 is guided onto the surface of a photosensitive drum serving as a surface to be scanned 78 through an fθ lens system (i.e. a scanning optical system) 76 having fθ characteristics, while the light deflector 75 is rotated in the direction indicated by arrow A to scan the surface of the photosensitive drum 78 in the direction indicated by arrow B (i.e. the main scanning direction) with the light beam.
Optical scanning apparatuses are ordinarily equipped with a turn back mirror(s) for the purpose of size reduction or registrational adjustment etc. FIGS. 11 to 14 are cross sectional views, each showing the principal portion of an image forming apparatus. FIG. 11 shows an arrangement in which the optical scanning apparatus 200 is not equipped with a turn back mirror. FIGS. 12 to 14 show arrangements in which at least one turn back mirror is provided in the optical scanning apparatus 200 to fold an optical path.
The image forming apparatus shown in FIG. 11 is disadvantageous in terms of size reduction and cost saving, since there are many useless spaces (the hatched portions) in the apparatus and many scanning lenses are required to shorten the optical path length. Generally speaking, as the number of the turn back mirrors increases, the freedom of arrangement of the optical path increases and the apparatus as a whole can be made more compact, though this is not always the case and the situation may change depending on the configuration of the parts in the optical scanning apparatus.
Conventionally, infrared lasers (with an oscillation wavelength of 780 nm) or infrared lasers (with an oscillation wavelength of 675 nm) have been used as semiconductor lasers serving as the light source means. However, optical apparatuses that can provide a small spot by means of a short-wavelength laser with an oscillation wavelength of shorter than or equal to 500 nm are under development to meet demands for higher resolutions. An advantage of the use of the short-wavelength laser is that it is possible to realize a spot diameter as small as approximately half the spot diameter attained in conventional apparatuses, while maintaining the F-number of the exit side of the scanning optical system as large as that in conventional apparatuses.
In addition, if the spot diameter is equal to that in conventional apparatuses, the F-number of the exit side of the scanning optical system can be made approximately twice as large as that in conventional apparatuses. Thus, the depth of focus at the surface of the photosensitive drum is greatly increased. (Note that the depth of focus is proportional to the wavelength of the light emitted from the light source and to the square of the F-number of the exit side of the scanning optical system). Accordingly, the cost can be reduced by allowing a decrease in the degree of accuracy of various parts or by eliminating an adjustment mechanism that is necessary in conventional apparatuses.
FIGS. 15 and 16 are cross sectional views in the main scanning direction (or main scanning cross sectional views) showing the principal part of optical scanning apparatuses in which a spot having a diameter of 60 μm with respect to the main scanning direction is formed using an infrared laser 81 with an oscillation wavelength λ of 780 nm and a blue-violet laser 1 with an oscillation wavelength λ of 405 nm respectively.
FIGS. 17 and 18 are graphs representing the depth of focus in the arrangements shown in FIGS. 15 and 16 respectively. In each graph, depth curves for a slice level of 75 μm are drawn with the horizontal axis representing the image height and the vertical axis representing the optical axis direction (defocus direction) of the scanning optical system. As will be seen from the depth curves, the depth of focus can be greatly increased with the spot diameter unchanged, as the oscillation wavelength of the light source is made shorter.
If recording of image information is to be performed at a high degree of accuracy with the above-described optical scanning apparatus, it is necessary that the curvature of field be excellently corrected all over the surface to be scanned, distortion characteristics with constant velocity characteristic (fθ characteristics) be established between the image angle θ and the image height, and the spot diameter on the image surface be constant at different image heights. As mentioned above, FIGS. 15 and 16 are main scanning cross sectional views of the optical scanning apparatuses in which the spot size is set to 60 μm with an infrared laser and a blue-violet laser respectively, and it will be seen that the beam width is smaller in the optical scanning apparatus shown in FIG. 16 in which the blue-violet laser with the smaller wavelength is used.
In the optical scanning apparatus using the blue-violet laser, it is possible to increase the depth of focus as compared to the apparatus using the infrared laser and cost reduction can be achieved by decreasing the degree of accuracy etc., but such an apparatus is sensitive to influences of scratches, dusts or scattered toner present on the surface of optical components since the width of the light beam is narrow.
FIG. 19 illustrates a scratch, dust and toner present on the surface of a turn back mirror. FIG. 20 shows a printed image that has been formed using such a turn back mirror. Since the laser (or the light source) is normally operated while image writing is performed, if the light beam is partially blocked by dust etc., the image density will become low at the corresponding position, and a white streak can be produced in the worst case.
As is well known, the shorter the wavelength of a light beam is, the more strongly the beam is affected by dispersion. When particles of several microns such as toner particles are adhering on the surface of the mirror, the phenomenon that a portion of the scanning light is prevented from reaching the surface of the photosensitive drum by dispersion may occur.
Therefore, employment of the blue-violet laser directly leads to the problem of unevenness in density of an image caused by small particles such as toner particles adhering on the mirror. In the case of a color image forming apparatus utilizing such an optical scanning apparatus, the problem appears as unevenness in color in an image. Therefore, in the optical scanning apparatus using a blue-violet laser, it has been necessary to take care of scratches, dust and the like more closely than usual, which leads to an increase in the production cost.